serdev: fix memleak on module unload

Make sure to free all resources associated with the ida on module
exit.

Fixes: cd6484e1 ("serdev: Introduce new bus for serial attached devices")
Cc: stable <stable@vger.kernel.org>	# 4.11
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serdev/core.c

@@ -617,6 +617,7 @@ EXPORT_SYMBOL_GPL(__serdev_device_driver_register);
 static void __exit serdev_exit(void)
 {
 	bus_unregister(&serdev_bus_type);
+	ida_destroy(&ctrl_ida);
 }
 module_exit(serdev_exit);